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I'm self-building a small timber-framed structure (essentially a 2x6 stud wall which will be externally sheathed and clad), which will include some UPVC double-glazed windows.

I'm aware that the window aperture should be prepared before installation with some sort of flashing to give a waterproof seal but it's unclear what exactly is needed. There are so many types of tape out there and every video I look at online uses something different - and probably half of them are not using the correct thing anyway!

I have seen products such as "Flash Tape" which are described as for flashing repairs in roofing but don't mention windows. I've equally seen people just use generic "waterproof tape"

It's not clear what I am actually looking for and googling is showing me a wide array of clearly different products, none indicating window installation as a use case.

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  • There are a wide variety of rubber tapes and sealants that can be used. Heck, you could use lead sheeting and just hammer it into shape. What have you learned (beyond "there are a lot of options") in watching all these videos that is still leaving you perplexed? We're not going to recommend a specific brand for you. Also, where are you located? In the US "timber-framed" usually means whole logs (or significant portions thereof), while in the UK (and other parts of the world), "timber-framed" usually means "built of dimensional lumber". That may be an important distinction.
    – FreeMan
    Commented Aug 19, 2022 at 12:18
  • What is the wall in your timber frame structure made of? Stress-skin panels are common, but a timberframe wall could be anything, and you don't say...
    – Ecnerwal
    Commented Aug 19, 2022 at 12:25
  • I've added a bit more detail but think "wooden shed". @FreeMan product recommendations aren't allowed but it's entirely reasonable to mention a real-life product as an example of what is typically used. I don't know what properties said material needs or what it is referred to.
    – Mr. Boy
    Commented Aug 19, 2022 at 13:03
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    "the window rests on a horizontal surface" That should not be true. That surface should be sloped out of the building a few degrees, as well as being flashed. It's also important that the bottom of the window not be caulked, so that water can drain out of the sill area if it gets in.
    – Ecnerwal
    Commented Aug 19, 2022 at 14:31
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    No, @Mr.Boy, the framing itself may be required to be slightly canted to the outside of the building. This helps any water that may manage to make it behind the multiple layers of defense can drip outward instead of puddling or worse, dripping inward. You waterproof in multiple layers of defense, just in case. If this is required or if you want to add it in as an extra precaution (nothing should prevent you from exceeding code), you would tip the whole board, not cut a chamfer into it. You want the framing to be full size.
    – FreeMan
    Commented Aug 19, 2022 at 15:32

2 Answers 2

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I've been a fan of This Old House for ages, so it's a bit of my go to for how to do things very right. Since they are watched by a large audience all around the US (and probably elsewhere in the world), it seems that they go overkill to exceed building codes, expecting that people will probably shortcut some of their methods, yet hoping that despite the shortcuts, their work will still meet local code. (At least that's what I believe, I have no empirical proof that this is the case.)

Step 3 from this This Old House website link:

Cut a 6-inch-wide strip of self-adhering waterproof membrane (or a 9- to 12-inch-wide strip of 15-pound builder's felt) 18 to 24 inches longer than the window's width. Center the membrane under the rough opening and adhere it to the existing builder's felt or house wrap. Make sure its top edge doesn't extend above the edge of the opening.

Cut two more strips of membrane (or felt) 1 foot longer than the height of the opening. Center and attach them along each side of the opening, overlapping the strip under the window.

Cut another strip of membrane (or felt) 1 foot longer than the window is wide; center and attach it across the top of the rough opening so that it overlaps the two side strips.

TIP: When applying flashing, layer the material so that any water running down the wall is directed out: Seams should never face up.

Note that the "tape" in this case is a self adhering waterproof/rubber membrane and not what one generally thinks of as "tape" (i.e. not something to hold a box closed).

Of course, it can be done without any "tape" at all, using roofing/builders felt (felt soaked in asphalt, making it highly water resistant) if that is allowed by the building codes in your jurisdiction.

You would need to find some sort of a self-adhesive membrane available near you. Any building supply store, and possibly a large DIY home-improvement store, should have something.

Long story short, you need to find a waterproof tape/membrane material that you can adhere to the rough opening, have some metal flashing for the top of the window, and you rely on the nailing flange of at the bottom of the window (attached at the factory) to be the "flashing" for the bottom of the window itself.

Just about any sort of waterproof membrane/tape will do the job. I have seen, on TOH, that they have used lead sheet to waterproof a window install on a recent show (something either in the most current season or the one previous - I don't recall the exact episode). So don't discount using that if necessary. Of course, you'll want to wash your hands before grabbing your sandwich for lunch, but once it's installed there's no danger from the lead because it will be buried under the window and whatever siding material you're using.

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  • yes, agreed that any membrane will do. I added an answer with my experience. (and +1 here)
    – P2000
    Commented Aug 19, 2022 at 18:19
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In combination with proper drip edging, I use tar paper.

With my first window installs I used tape, and found it more work, cost and hassle than necessary.

I now silicone the window fin to the paper, and it has been water tight in a very rainy climate.

Proper lapping is key: work from bottom up, lap 2 or more inches on horizontal seams (special care over the top window fin), and 4 or more inches on vertical seams.

The drip edge should clear the top of the window. And lap the tar paper over the top flange of the drip edge. Your window installation instructions likely also details these critical steps.

No expensive gue or glue is as good as proper lapping and edging, over a 20+ year period.

Staple sparingly to hold paper in place until window, trim and siding are installed.

I do not seal the trim, and instead let water seep through and let it air dry. Seals can lead to trapped water which can cause rot, or separation in freeze-thaw cycles.

The trim and siding must, however, entirely cover the paper to protect it from UV, which would otherwise deteriorate its strength and water repellant property.

If you live in an excessively stormy region, e.g. North Atlantic coastal, your building code may require otherwise.

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